Nanny camera

I need to start going to the office from next month and so we are interviewing prospective nanny's. We have a few shortlisted and there are more interviews scheduled. i don't want my girl to be in the hands of an abusive nanny. We are planning to have a nanny cam ( http://www.goodalarmbadalarm.com/benefits-of-a-na… ) installed. My doubt is should we tell the nanny about this being installed so that it prevents them from being abusive.